Carrier aggregation (CA) is one of the key technologies in Release 10/11 (Rel-10) of the 3rd generation partnership project (3GPP), and is used to increase the throughput for user equipment (UE) to suit the increasing demands for higher the data rates. Inter-frequency band carrier aggregation and intra-frequency band carrier aggregation are both supported by the CA UEs. For carrier aggregation, there are two kinds of cells called primary cells (PCells) and secondary cells (SCells). The UE will first access the PCells via legacy procedure (e.g., a procedure in accordance with 3GPP Release 8) and after that, once the evolved Node B (eNB) sees the need to extend the throughput, it could configure one or more SCells for the UE.
However, for UE configured with carrier aggregation, power consumption performance need to be considered. Quick SCell activation/deactivation was introduced in 3GPP Rel-10. When a UE has a large amount of data to transmit, the eNB can activate the SCell for radio resource extension; when the UE has less data to transmit, the eNB can deactivate the SCell to save UE power. An eNB could thus use an activation/deactivation media access control (MAC) control element (CE) to activate/deactivate the SCell. In addition, the UE also includes an implicit deactivation method (e.g., sCellDeactivationTimer), which can deactivate the SCell in case the MAC CE is lost at the UE and there was NACK to ACK error.
Accordingly, SCell activation and deactivation has become an area of increasing focus. In particular, issues have emerged as a result of variable activation timing of SCells. In 3GPP Rel-10, it was assumed that SCells could be activated relatively quickly. For example, if a MAC CE command implying activation was received on subframe n, then it was assumed that the activation of the SCell would be complete by subframe n+8.
However, as more detailed work has been performed, there are more challenging cases where user equipment (UE) algorithms (such as automatic gain control (AGC)) could cause activation of the SCell to take longer than this duration. Other examples of more challenging cases are when there is a time-division duplex (TDD) configuration which has mostly uplink subframes, or for frequency division duplexing (FDD), when multi-broadcast single frequency network (MBSFN) subframes occur around the activation time. An additional issue that was discovered was the so-called “blind” activation case, where the eNB configures and immediately activates an SCell which has not been previously measured by the UE. In this case, the UE needs to perform primary synchronization signal (PSS)/secondary synchronization signal (SSS) timing acquisition. As a result of these potential delays in SCell activation, standards discussions have moved in the direction of specifying generic minimum performance requirements for SCell activation, although in many cases it is likely that a UE could activate the SCell more quickly than the generic minimum performance requirement.
As mentioned previously, during the initial 3GPP Rel-10 discussion, it was agreed that a UE should finish the activation/deactivation procedure in 8 milliseconds (ms), meaning that if the UE receives an SCell activation/deactivation MAC CE at subframe n, the SCell should be ready at subframe n+8. However, some commentators have noted that 8 ms is not enough for every practical use case. However, lengthening the expected activation time will potentially reduce the likelihood of using activation/deactivation to control UE power consumption. Moreover, lengthening the expected activation time could cause unnecessary idle time when the SCell is ready more quickly without knowledge of the network. Accordingly, better communication of actual SCell activation/deactivation times could mitigate these issues, reduce idle downtime, and thus enhance system performance.